IPTC 16579

Fundamental approaches to dolomitization and carbonate diagenesis in different hydrogeological systems and the impact on reservoir quality distribution

Yitian Xiao1, Gareth D Jones

1, Fiona F Whitaker

2, Anwar B Al-Helal

2, Sherry Stafford

3, Enrique Gomez-Rivas

4,

Sean Guidry3

1ExxonMobil Exploration Company, Houston TX 77002, USA

2Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK

3ExxonMobil Upstream Research Company, Houston TX 77252, USA

4Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 26–28 March 2013. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435

Abstract

The accurate prediction of the geometry of subsurface dolomite geobodies, their connectivity, and the distribution of

reservoir properties is a fundamental challenge in carbonate reservoir characterization. Reactive Transport Models (RTM)

couple geochemical reactions with fluid flow to facilitate both 2D and 3D quantitative, process-based investigations of

dolomitization and related carbonate diagenetic reactions. The paper will highlight new results and key conclusions from

simulations of dolomitization mechanisms in four different hydro-geological systems: 1) Brine reflux, 2) Mixing zone and

sub mixing zone, 3) Geothermal circulation and 4) Fault controlled hydrothermal circulation. Simulations provide new

insights on the spatial distribution and dynamic behavior of:

Geometry and distribution of dolomite bodies generated by different styles of subsurface fluid flow and their

dynamic interactions;

Regional versus local controls on dolomite occurrence and connectivity;

Sensitivity and hierarchy of geological controlling parameters;

Spatial and temporal relationships between dolomitization and associated diagenetic minerals including anhydrite

cements and Mississippi Valley Type (MVT) mineralization;

Effect of hydrothermal fluid induced dolomite recrystallization and anhydrite dissolution;

Criteria to help identify the distribution of reservoir quality including high permeability dolomite “sweet spots”.

When integrated with conventional subsurface data and stratigraphic, geochemical, and structural framework, Reactive

Transport Models of dolomitization provide fundamental and robust predictive concepts and reservoir quality models for

exploration and new / mature field developments. In particular, the state-of-the-art simulations allow the analysis and 3D

visualization of dolomite body spatial and temporal evolution that can translate into alternative “process-based” well

correlation methods and strategies for populating diagenetic bodies and their petrophysical properties in geological models

for reservoir flow simulations.

2 IPTC 16579

INTRODUCTION

The spatial distribution of dolomite presence and in particular the diagenetic transitions between limestone and dolomite are

often a critical indicator on reservoir quality in many carbonate reservoirs [1-2]. The Jurassic Ghawar field in Saudi Arabia is

the most notable prolific Middle East examples. Depending on original depositional texture, style of dolomitization and

presence of anhydrite cement, dolomite can behave as a reservoir or as a baffle / barrier to flow [1-2]. High permeability, or

so-called “Super K” dolomites with multi-darcy permeability that sporadically occur in thin zones (sometimes <5 ft) are vital

contributors to individual well and overall reservoir performance [2]. Laterally continuous low permeability ‘tight” dolomites

can also have significant impact on subsurface flow and thus reservoir management strategies. The transition between

limestone and dolomite is readily identified in both wireline logs and core and is dominantly observed as a sharp interface

described as a “dolomite front” sensu Wilson et al. [3]. Unfortunately, dolomite fronts in carbonate reservoirs are often below

seismic detection, particularly those associated with ‘thin’ beds of dolomite on the order of a few to several tens of feet thick.

Predicting the spatial distribution or connectivity of dolomite and associated petrophysical properties between wells and

away from well control is thus a fundamental challenge in carbonate reservoir characterization. In contrast to depositional

facies there are no defined rules or guidelines for correlating dolomite bodies in the subsurface. Existing correlations of

dolomite tend to be lithostratigraphic and contain considerable uncertainty in the degree of interpreted reservoir geobody and

connectivity (Figures 1-2).

One way to understand the distribution and connectivity of sedimentary bodies is to study outcrop analogs [4-9]. Many

outcrop studies have focused on dolomitized carbonates. However, there are few predictive dolomitization models and

among them very few have specifically focused on transitions between limestone and dolomite and / or described the

connectivity of dolomite bodies. Outcrop based studies by Wilson et al [3] and Yao and Demicco [7] that mapped dolomite

fronts associated with geothermal circulation and ascending hydrothermal fluids, respectively, are notable exceptions. These

analogs are, however, not appropriate for reservoirs where the majority of dolomite is interpreted to have formed by the

reflux of brines from evaporated seawater, Ghawar field in the Middle East [2] for example. Dolomite fronts mapped in Plio -

Pleistocene outcrops on Bonaire by Lucia and Major [8] are interpreted to have formed by brine reflux (Figure 3). The

Bonaire study provides insights on the propagation of a dolomite front and the distribution of petrophysical properties in a

reflux system but is of limited use in helping develop dolomite correlation strategies in extensive carbonate ramp reservoirs

like Ghawar field. Other smaller scale outcrop examples of limestone dolomite transitions interpreted to have formed as a

consequence of brine reflux are shown in Figure 4.

Figure 1. Correlations of dolomite distribution and flow percent in the Arab-D Formation in Ghawar Field, Saudi Arabia. After

Cantrell et al. [2].

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Figure 2. Correlations of limestone to dolomite (shaded grey) transitions from wireline well logs in the Ghawar Field, Saudi Arabia.

Note 1-2 km well spacing and vertical scale in ft. After Cantrell et al. [2].

Figure 3. Limestone to dolomite transition mapped in a Plio Pleistocene outcrop on Bonaire, Netherlands, Lesser Antilles. Note

dolomite ‘fingering’ in direction of depositional dip. Modified after Lucia and Major [8].

Figure 4. Outcrops of limestone dolomite transitions. (A) Devonian, Cripple Creek, Canada; (B) Ordovician, Franklin Mountains,

USA, after Tillotson [10]; (C) and (D) Ordovician, Franklin Mountains, USA (Images courtesy of Jerry Bellian). In all images

Dolomite is tan (light brown) and limestone is grey. Red arrow shows interpreted direction of brine reflux paleo fluid flow.

4 IPTC 16579

CONCEPTUAL MODELS OF DOLOMITIZATION

It is well established that large-scale dolomitization requires both the active circulation of Mg-rich fluids (commonly derived

from seawater) and favorable geochemical conditions to overcome kinetic barriers to dolomitization. Machel [4] and

Whitaker [10] recognized that most models for the formation of replacement dolomites are essentially hydrological models.

Flow of dolomitizing fluids may result from gradients in density generated by contrasts in salinity (brine reflux [14]) or

temperature (geothermal convection [11-13]), or pressure gradients generated by elevation head, tectonic or sedimentary

compaction [15], or fault induced hydrothermal fluid flow [4]. Early hydrological work suggested that these different flow

systems might generate distinctive distributions of dolostone [3, 4, 10]. However, flow of diagenetic fluids is critically

dependent on spatial variation in permeability, generating complex dolomite geobodies at a range of scales. In addition,

whilst simplistic models of individual flow systems are conceptually valuable, hybrid circulation systems commonly develop

where more than one flow system operates simultaneously [13]. Finally, natural flow systems will evolve over time in

response to changes in rock properties and/or boundary conditions, such as relative changes in sea level, or the feedback to

the fluid flow from the altered rock properties [14]. Meaningful prediction of fluid flow systems capable of dolomitization

thus depends on capturing such complexities [13].

Figure 5. Conceptual hydrological models of dolomitization. After Wilson et al [3], Machel [4].

Reactive transport models that explicitly couple flow and chemical reactions facilitate investigations of the distribution of

diagenetic alteration, including dolomitization, in the subsurface [10-19]. Numerical models have the advantage of being able

to simulate the specific system of interest and to track the spatial and temporal evolution of diagenesis in contrast to the

terminal diagenetic overprint observed in outcrop studies [10-19]. Linking fundamental geochemical processes to predict

reservoir quality has significantly improved our understanding of several conceptual diagenetic models and paradigms. Our

novel approach suggests that reactive transport models, when sufficiently integrated with traditional methods and calibrated

with field data, have the potential to significantly improve our ability to predict carbonate and siliciclastic reservoir quality

[10-19].

In this paper we present results from reactive transport models employed to investigate dolomitization mechanisms in four

different hydro-geological systems: 1) Brine reflux, 2) Mixing zone and sub mixing zone, 3) Geothermal circulation and 4)

Fault controlled hydrothermal circulation. The RTM results provide new insights on the distribution of dolomite, anhydrite

and implications for reservoir connectivity and subsurface correlation strategies.

IPTC 123456 5

NUMERICAL MODELS AND APPROACH

A reactive transport model relies on a mathematical formulation to describe geochemical processes involving fluid-rock

interactions. The general governing equation can be written as:

kk

iii

it

C

x

Cv

x

CD

xC

t

)( (1)

Where Ci is the concentration of a specific species in the pore fluid, D is the combined diffusion and dispersion coefficient

term, v is the linear fluid flow rate, and ф is the porosity. The first two terms on the right hand side describe the transport

process (diffusion, dispersion, and advection) while the last term describes the effect of geochemical reactions (Figure 6).

Figure 6. Schematic diagram showing the reactive transport process involving dolomitization.

For example, the reaction of dolomitization can be expressed as:

26.2)1(

eq

RT

E

dolo

K

QeAS

t

C a

(2)

Where S represents the reactive surface area, A is the rate constant, Ea is the activation energy, Q/Keq is the saturation index,

and 2.26 is the reaction order [20].

Due to complex boundary conditions and complicated coupling between the transport and reaction terms, it is impossible to

provide analytical solutions to equation (1) for even the simplest geochemical system. Therefore numerical solutions have to

be used. Fortunately, due to the exponential increase in computational power, realistic reactive transport models have been

developed that are beginning to provide exploitable insights to reservoir quality prediction. We have used two commercially

available numerical programs, Xt2 [21] for the 2D models and ToughReact [22] for the 3D models. The rest of this paper

highlights results from our recent efforts in applying this emerging technology to the quantitative prediction of the spatial and

temporal distribution of dolomitization and its effect on carbonate reservoir quality and connectivity.

RESULTS

Geothermal Convection Model

Geothermal (‘Kohout’) convection of seawater cools the margins of carbonate platforms, warming the interior at shallow

depth, and has been invoked to explain dolomitization during early burial. Geothermal convection can occur as “free” or

“forced” convection. Free convection is characterized by counter-rotating fluid flow cells which develop in anisotropic

carbonate platforms [11-13] or those buried by low permeability sediments [10]. However, a substantial thickness (3-500 m)

free of laterally continuous mudstone / shale beds is required for free convection to occur. Furthermore, unless these free

convection cells are partially hydraulically open (“mixed convection”), the available magnesium would rapidly be utilized

and dolomitization would cease. In contrast, forced convection of cold seawater through carbonate platforms during early

burial (also termed “Kohout convection”) draws on an infinite reservoir of magnesium. Forced convection affects large

volumes of carbonates for extended time periods and appears essentially independent of relative sea-level [11-13].

Previous short (0.1 my) RTM simulations [11] suggested that this process can drive replacement dolomitization and

anhydritization, mostly at >50oC, but complete dolomitization would require more than 70 my (Figure 7). Our RTM

6 IPTC 16579

simulations over a more extended time frame (up to 30 my) using the same hydrogeological and geochemical

parameterization indicate significant non-linearities in the system, consistent with high-temperature experimental results.

Geothermal dolomitization may completely dolomitize parts of the platform in around 15 my (Figure 7). As dolomitization

proceeds, the process becomes predominantly flux controlled, with development of a wedge-shaped dolomite body which

thins from the platform margin to the interior and is formed at considerably shallower depth and cooler temperatures (20-

30oC) than suggested by extrapolation of short simulations.

Figure 7. Distribution of dolomite as fraction of rock volume predicted from RTM. 1) Early short simulation (100 ky) results from

Wilson et al [11]; 2) Long-term baseline simulation at time steps from 0.1 to 30 my. Note 10 times adjustment in scale bar from

0.1–2.5 my displays to those for 5-30 time steps my (after Whitaker and Xiao, [12]).

Dolomitization is relatively insensitive to changes in boundary conditions such as relative sea-level and platform geometry,

but is significantly slower in circular rather than more elongate platforms. Sediment permeability and reactive surface area,

often inversely related, are key controls on dolomitization. The restricted fluid flux limits geothermal dolomitization to the

platform margin of low permeability muddy platforms despite high reactive surface area. Dolomitization in more permeable

grainy platforms is limited by the reactive surface area, and occurs only in the platform core due to widespread cooling.

Vertical layering of sediment types produces a complex vertical diagenetic stratigraphy, favoring more reactive beds at

shallow depth where permeability is not limiting, but switching to more permeable beds at depth. Bank-marginal fracturing

limits dolomitization of the platform interior, irrespective of whether the fractures are baffles or conduits for flow (Figure 8).

Figure 8. Comparison of distribution of dolomite as fraction of rock volume from a 1) homogeneous platform; 2) fractured

platform margin; 3) muddy layers in platform; 4) grainy layers in platform (after Whitaker and Xiao [12]).

IPTC 123456 7

Mixing Zone Model

The “mixing zone” model of doolomitization is perhaps the most controversial. In this model the mixing of seawater and

freshawater theoretically results in a fluid that is super saturated with dolomite and undersaturated with respect to calcite [see

review in 4]has raised the most questionsIn the 1990’s a series of qualitative predictive concepts were developed that related

different early carbonate diagenetic processes and products to sequence stratigraphy [23-25], platform type and climate

(Figure 9; see summary in Moore, 2001, p. 245-291 [26]). These “cartoon” models illustrate conceptually how the spatial

distribution of diagenetic styles is related to different hydrological zones in a carbonate platform. However, these overly

simplistic models are inadequate for making any quantitative diagenetic predictions, in particular about the feasibility of

significant dolomite formation.

Figure 9. Conceptual model of an emergent isolated carbonate platform (low stand systems tract) showing the distribution of

different hydrological zones (after Moore, 2001 [26]).

2D RTMs were developed to investigate early diagenesis in an isolated platform. Figure 10 shows RTM results including the

simulated fluid flow and hydrological zone (vadose zone, freshwater lens, mixing zone, and saline zone) distribution in an

isolated carbonate platform. In the salinity distribution map (Figure 10A), the blue portion represents vadose zone and

freshwater lens, the yellow part represents the mixing zone, and red portion represents seawater. The horizontal red line

represents the sea level, and the arrow line is the streamline representing the flow direction of the fluid. The vadose zone is

about 4 meters thick across the platform. The thickness of the freshwater lens varies, and is deepest at about 15 meters in the

middle of the platform. The mixing zone appears to be thinner in the middle of the platform and thicker at the platform

margin. The overall hydrological zone distribution is in excellent agreement with other simulation results (with the same

boundary conditions) as well as with the Ghyben-Herzberg model.

Figure 10. Simulation results of fluid flow and hydrological zone distribution in an isolated carbonate platform. A) Salinity; B)

Flow rate. All distance units are in meters and flow rate units are in m/yr.

The simulation results suggest that due to the lack of Mg2+

and slow reaction rates, mixing zone dolomitization is not a viable

mechanism for large-scale dolomitization in isolated platforms (Figure 11). As a matter of fact, due to non-linear mixing

behavior of the thermodynamics, mixing zone fluids are often undersaturated with respect to calcite and dolomite and can

8 IPTC 16579

lead to significant dissolution [15]. Circulation of seawater in the sub-mixing zone, on the other hand, might result in more

dolomitization (up to 3-5 % in 100 ky) than previously thought. This is a good illustration of how reactive transport models

can be used to evaluate proposed conceptual models like the mixing zone and identify / support alternative dolomitization

models in the same hydrological system.

Figure 11. Simulation results showing potential early dolomitization in an isolated carbonate platform after 100 ky.

Reflux Model

2D Simulations of Brine Reflux Dolomitization

Reflux dolomitization is widely accepted as a key mechanism for many large scale dolomitization and therefore has been

subject to numerous RTM studies. We developed a 2D model based on the reflux model originally proposed by Adams and

Rhodes [23] to explain dolomitization of Permian carbonate in West Texas, which shows a close stratigraphic association

with platform interior evaporites. The reflux model has been modified by several researchers [23-30] to incorporate different

geological scenarios. To capture the heterogeneous porosity and permeability of carbonate sediments in the shallow

subsurface (0-500 m), due to either variations in sediment texture or early diagenesis [29-31], we incorporated heterogeneous

porosity and permeability distributions in our 2D models to investigate the effect of heterogeneity on reflux dolomitization.

The simulation results demonstrated that the initial porosity and permeability heterogeneity in the precursor sediment had a

significant effect on the pattern of dolomitization (Figure 12). The applied heterogeneity results in the formation of

pronounced perturbations, in the geometry of the dolomite front, which extended several km basinward from the main

dolomite body after 2 My of active reflux (Figure 12 A-E). These perturbations, described as dolomite “fingers” by Wilson et

al. [3], became more pronounced with time suggesting a positive feedback between the initial exploitation of preferential

flow paths and subsequent changes in rock properties by dolomitization that enhance flow (Figure 12 A-E) as well as

negative feedback associated with anhydrite cementation (Figure 12 F-J).

The simulation results indicate the complex association of anhydrite cementation and dolomite distribution (Figure 12 F-J).

The initial anhydrite cement was formed due to the interaction of SO42-

from the brine with the excessive Ca2+

released from

the limestone due to dolomitization. However, as the dolomitization completed “upstream”, no more excessive Ca2+

was

released, which led the brine dipping below gypsum (anhydrite) saturation. This is the primary reason for the remobilization

of the initially formed anhydrite cementation (Figure 12 F-J). Focusing of reflux flow resulted in enhanced anhydrite

dissolution in the dolomite fingers, whereas anhydrite had a greater preservation potential adjacent to the fingers (Figure 12

J). Dolomitization enhances porosity by up to 8 % and anhydrite cements occlude porosity by up to 14 % (Figure 12 K-O).

The dolomite fingers were porosity “sweet zones” surrounded by non- to partially dolomitized sediments that were locally

cemented with anhydrite and have a lower porosity (Figure 12 K-O).

The 2D reactive transport simulations demonstrate the complex and dynamic nature of dolomitization associated anhydrite

cementation and porosity evolution in a simplified reflux system. Incorporation of initial porosity and permeability

heterogeneity resulted in complex dolomite geometry with pronounced dolomite “fingers” extending ahead of the main

dolomite body. The simulated dolomite distributions seem to change from thick and extensive to thin and separated in the

direction of the reflux flow, consistent with outcrop observations [Figures 3 and 4]. The distribution of limestone to

dolomitization transition and potential location of reservoir “sweet” spots can have important implications on reservoir

connectivity. For example, the extensive main dolomite body in the platform interior (Figure 12 E) suggests good lateral and

vertical reservoir connectivity, whereas in the limestone to dolomite transition zone the layered dolomite “fingers” and

IPTC 123456 9

anhydrite cement indicates complex reservoir connectivity, with good reservoir intervals (uncemented dolomite) sandwiched

by tight zones with extensive anhydrite cementation (Figure 12 J, O).

Figure 12. Effect of initial porosity and permeability heterogeneity on dolomite, anhydrite and porosity (A-E) Dolomite (contours

are percent dolomite with a contour interval of 5); (F-J) anhydrite (contours are percent anhydrite with a contour interval of 2);

and (K-O) porosity (contours are porosity [fraction] with a contour interval of 0.02). After Jones and Xiao [14].

3D Simulations of Brine Reflux Dolomitization

One key requirement for geological models developed for simulating production of hydrocarbons and reservoir management

is the 3D distribution of petrophysical properties. 2D reactive transport models of dolomitization are limited by their ability

to incorporate spatial variations in natural variables that control brine reflux and thus extrapolation to the distribution of

dolomite in 3D. We employed 3D reactive transport models to investigate the effect of permeability and spatial distributions

in the magnitude and distribution of platform-top brines that source subsurface brine reflux. Salinity variations observed in

modern environments, for example Lake Macleod [32] in addition to those interpreted from the thickness and type of

evaporite deposits in the rock record were used to condition model boundary conditions and initial rock and fluid properties.

We specified a flow domain 5 km x 5km x 100 m with a grid node spacing of 200 m horizontally and 2 m vertically. Reflux

was simulated from three platform-top brine ponds approximately 500 m in diameter with different maximum salinities of

210, 175, and 140 ppt (Figure 13) The basal boundary was specified as a no flow boundary (refluxing brines never reached

this depth in the geological time simulated) and the sides of our flow domain cube were specified as hydrostatic and thus

open to flow. All other parameters and boundary conditions were specified as those used in our 2D reactive transport models

of brine reflux that incorporated a heterogeneous distribution of permeability described above in Jones and Xiao [14].

The RTM results showing the 3D evolution of brine reflux up to 5000 years are depicted in Figure 13. Individual reflux brine

plumes sourced by the three different ponds, evident after 500 yrs of flow, eventually coalesce to form a single brine plume

(Figure 13). At 5000 years the mesosaline to hypersaline brine plume extends over the 25 km2 area modeled to a maximum

depth of approximately 50 m but with considerable variation in vertical extent related to brine pond proximity and local

permeability heterogeneity. Modeled maximum rates of brine reflux were on the order of few meters per year with spatial

variations controlled by the proximity to the different brine ponds and permeability. The resultant flow field is the net product

of both reinforcing and opposing flows from the three separate ponds. In contrast to uni-directional flow in our 2D results, 3D

simulations demonstrate that refluxing bines, at least locally in the vicinity of brine ponds, have the potential to flow up and

down depositional dip and along depositional strike. After approximately 5000 yrs the brine plume appeared to reach a quasi-

steady state relative to the effective hydraulic head in the brine ponds that drives flow. Beyond 5000 years advection is

limited and the slower process of diffusion controls brine plume spreading. It is unlikely that steady state reflux would be

obtained in nature due to variations in climate, sea level and geomorphology that act at shorter time scales to change the

distribution of platform-top salinity that drives flow.

The 3D simulations generated more complex patterns of brine reflux and distributions of dolomite and anhydrite than

observed in our 2D study and has significant implications for understanding connectivity in carbonate reservoirs (Figures 14

and 15).

Dolomite Anhydrite Porosity(A) 0.2 My

(B) 0.32 My

(C) 0.56 My

(D) 1 My

(E) 2 My

(F) 0.2 My

(G) 0.32 My (L) 0.32 My

(H) 0.56 My (M) 0.56 My

(I) 1 My (N) 1 My

(J) 2 My (O) 2 My

(K) 0.2 My

10 IPTC 16579

Figure 13. 3D Simulation showing the distribution of reflux brines at 50, 100, 200, 500, and 5000 yr. Sea water in black, hypersaline

brines in yellow and orange, and mixed fluids in blue.

3D Evolution of Dolomite - ‘Main Body’ and ‘Fingers’

The spatial evolution of dolomite from the RTM over a period of 100 ky for the reflux brine plume depicted in Figure 14,

demonstrates how initially small isolated dolomite bodies located beneath and adjacent to the brine ponds grow and

amalgamate to form a single dolomite body that extends over the entire 25 km2

(Figure 14). Results show dolomite bodies

grow at different rates. In a similar manner to our 2D simulations the rate of dolomite body propagation is a function of the

Mg2+

concentration of the brine and the flow rate. In a reflux system this is controlled by the salinity of the brine ponds,

distance from the pond and the magnitude of permeability (assuming a uniform temperature and reactive surface area).

Simulated dolomite body growth occurs by the propagation and eventual amalgamation of dolomite ‘fingers’ (Wilson [3]).

The reaction front from limestone to dolomite is sharp. Visualization of the 3D dolomite body after 100 ky of reflux shows

the significant variation in the dimensions and shape of dolomite “fingers” that extend beyond the “main” dolomite body,

here described as dolomite that is laterally continuous over the areal extent of the flow domain. Dolomite fingers thin in the

direction of flow and tend to become more pronounced with time depending on the specified feedback of both dolomitization

and anhydrite cementation on permeability and the initial permeability of limestone the dolomitization front intersects along a

reflux flow path. Examination of model results using 2D slices shows dolomite bodies that are apparently isolated from the

main body (Figure 14).

Figure 14. RTM 3D distribution of dolomite: (A) Dolomite evolution from 20 to 100 ky; (B)-(C) 2D slices showing the dolomite

main body and fingers in x- and y-directions; (D) 2D slices showing dolomite in z-direction. Dolomite in red and limestone in blue.

IPTC 123456 11

However, these bodies are connected to the main body in 3D and were generated by the transport of Mg2+

along a flow path

out of the 2D plane visualized (Figure 14). Thus reflux associated with local brine ponds has the potential to generate

complex distributions of dolomite with ‘finger’ orientations that reflect multi directional fluid flow. This has three important

implications for subsurface correlation and geological modeling of reflux dolomites: 1) Understanding the processes of reflux

and paleo fluid flow can provide a framework for correlation scenarios and guidelines for geological modelers 2) Reflux flow

can be locally multi directional with respect to depositional dip and strike and 3) Dolomite “fingers” tend to connect to the

“main body” in a single reflux episode and are rarely isolated “pods”. The occurrence of isolated dolomite pods is restricted

to the early termination of reflux as illustrated by the distribution of dolomite at times less than 60 ky (Figure 14), multiple

episodes of brine reflux separated in time and space and dolomitization by other mechanisms [11-14].

3D Evolution of Anhydrite Distribution (Bedded and Cements).

Dolomites generated in a reflux system have a multi-faceted relationship with evaporites that are depositional and / or occur

as cements [14]. Our 3D simulations of brine reflux dolomitization show two types of anhydrite: 1) Anhydrite cements that

precipitate ahead of a migrating dolomite front due to the release of calcium as explained in our 2D study [14] and 2) the

direct precipitation of “bedded” anhydrite from evaporated seawater (Figure 15). Bedded anhydrite is distributed as a laterally

continuous body (Figure 15) with rare isolated “holes” that may act as potential fluid conduits (Figure 15 D). Anhydrite

cement has a more complex distribution that is related to the distribution of dolomite. Greater preservation of anhydrite

cement is evident in the 3D models of dolomitization (compare Figures 12 and 15). This is due to the shorter duration of

brine reflux simulated and the precipitation of bedded anhydrites that reduce permeability, retard flow and slow rates of

anhydrite dissolution. Future simulations will explore the relationship between dolomitization and anhydrite and the

implications for identifying dolomite reservoir sweet spots where anhydrite cements are rare / absent.

Figure 15. RTM 3D distribution of anhydrite (bedded and cement): (A) Anhydrite evolution from 20 to 100 ky; (B)-(C) 2D slices

showing the bedded anhydrite (red) and anhydrite cement (green) in x- and y-directions; (D) 2D slices showing anhydrite in z-

direction. Thin section photos courtesy of Jim Markello.

High Permeability Dolomite Distribution

Understanding the distribution and connectivity of dolomite bodies in the subsurface is the first step towards building more

accurate reservoir models. The magnitude and distribution of permeability in a dolomite body is an equal if not more,

important consideration. Contrasts in dolomite permeability can be extreme and occur over short distances. The problem is

exemplified by the occurrence of so-called “Super-K” high permeability (> 100 mD to several D) dolomites that are often

thin (a few feet) such as those in Ghawar which account for a significant portion of total production [2]. Reactive transport

models provide a process based method of investigation the origin and spatial distribution of dolomite permeability (Figure

12) [14]. We interrogated our 3D model to illustrate reservoir sweet spot identification applying the following illustrative

criteria: Dolomite with > 15 % porosity and < 5% anhydrite (Figure 16). Dolomite “sweet spots” (shown in red) are located

close to the dolomite front, have a variable thickness and are generally well connected in this case (Figure 16). These sweet

spots occur in both the dolomite “main body” and “fingers” (Figure 16). This general pattern is similar, but not exclusive to

several dolomite reservoirs, although our model is a simplified hypothetical example (authors’ personal observation).

Permeability reduction via the overdolomitization process was not observed in our 3D model, perhaps due to the relatively

short simulation time. Chemical compaction, hydrothermal dolomitization and dolomite recrystallization could cause

overdolomitization. These diagenetic processes were not included in our simulations.

12 IPTC 16579

Figure 16. RTM 3D distribution of (A) dolomite; (B) Anhydrite; (C) High-permeability dolomite (dolomite with < 5% anhydrite

and > 15 % porosity). All properties are shown in 2D slices in x-direction. Porosity intervals are shown in contours and “high-K”

dolomite is shaded in red.

Hydrothermal Model

There are large volumes of hydrothermal dolomite (also called high-temperature dolomite - HTD) in oil-producing carbonate

reservoirs (Davies and Smith [33]). Therefore there is a need to understand how they were formed in order to make better

predictions. However, there is still a controversy on the origin of HTD [33-42]. Machel [4] stated that in many cases

hydrothermal dolomite only occurs as an overprint of earlier dolomitization phases, and it is not directly replacing original

limestone. However, others suggested that this is not the case for most hydrothermal dolomites [33].

2D RTM models were developed to investigate the effects of fault associated Mg2+

bearing hydrothermal fluids on carbonate

reservoirs. Figure 17 shows the distribution and brine, dolomite, anhydrite, temperature, and calcite after 100 ky. The results

suggest that with continuous hydrothermal flow over tens of thousands of years, late diagenesis can significantly alter the

reservoir properties and lead to large scale HTD formation. The results also suggest that depending on the initial rock

composition and timing of the HTD event, the reaction can lead to no anhydrite cement formation in a lower dolomite

interval (formed earlier), anhydrite formation in the middle limestone interval, and anhydrite dissolution in the upper

anhydritic dolostone interval (Figure 18). This is because the HTD fluid was initially in equilibrium with the lower dolomite

rocks and therefore no anhydrite cementation occurs, as the replacement dolomite takes place in the middle limestone

interval, the sulphate ions will interact with the excess Ca2+

to form anhydrite cement. As the HTD fluid enters the top

anhydritic dolostone interval, it is depleted in sulphate and Mg2+

ions and therefore could lead to anhydrite dissolution, a

potential mechanisms to enhance the permeability of dolomite that was plugged with early anhydrite cement. The conceptual

model shows how the RTM of HTD can help understand some basic questions – how far from a fault does HTD extend.

What’s the vaiations along a fault both strike and dip, etc.

A recent RTM investigate coupled with outcrop study [43] suggests that overpressured dolomitizing fluids released during

active fracturing under the form of fluid pulses, as expected in a syn-rift tectonic period, may result in large lateral flow rates

along the most permeable layers. However, these high fluid velocities are not maintained for periods longer than 2-5 years

according to the configuration of the Benicassim half graben. In contrast, a long-term fluid circulation system in a post-rift

environment can provide enough warm solution to the reactive shallow layers whenever a driving pressure and/or

temperature gradient is maintained over the time period in an open system. In this scenario, dolomitizing solutions flow

laterally along high permeability layers heating up host and surrounding rocks over long-time periods. This system must be

open in order to have enough Mg for significant dolomitization to occur. A difference in horizontal permeability of two

orders of magnitude is required to account for fluid flow in specific layers [43].

Anhydrite bedding

Anhydrite cement

Dolomite main body

Isolate dolomite fingers

High Permeability Dolomite

(A) (B)

(C)

IPTC 123456 13

Figure 17: RTM results showing the distribution and brine, dolomite, anhydrite, temperature, and calcite after 100 ky of fault

associated hydrothermal diagenesis.

Figure 18: RTM results showing HTD fluids entering a carbonate reservoir can lead to no anhydrite cement in the lower dolomite

interval, anhydrite cement in the middle limestone interval, and anhydrite dissolution in the top anhydritic dolostone interval.

CONCLUSIONS

Reactive transport models (RTM) to investigate key controls on reflux dolomitization and reservoir quality in four different

hydro-geological systems: 1) Brine reflux, 2) Mixing zone and sub mixing zone, 3) Geothermal circulation and 4) Fault

controlled hydrothermal circulation. The models predicted the spatial and temporal distribution of key geological parameters,

such as limestone, dolomite, anhydrite and porosity, and how these parameters were affected by depositional and diagenetic

variables (initial lithology, porosity and permeability, diagenetic fluid compositions, temperature and pressure, etc.). The 2D

and 3D modeling results were consistent with regional and reservoir trends in dolomite/anhydrite distribution published in the

literature and provided a better understanding of key diagenetic processes controlling reservoir quality in dolomitized

carbonate reservoirs. The modeling results, together with well-based observations, generated predictive diagenetic rules that

can be used to guide porosity and permeability distribution in 3D reservoir connectivity models.

14 IPTC 16579

The key conclusions and new insights from the reactive transport model include:

1. Reactive Transport Models have the capability to simulate the process of brine reflux dolomitization and the resultant

distribution of dolomite, anhydrite and petrophysical properties in 2 and 3D.

2. Sensitivity analyses revealed the hierarchical controls of natural variables (salinity, initial lithology, porosity,

permeability and heterogeneity, temperature, and residence time) on the distribution and evolution of the reflux system.

3. Forced convection of cold seawater through carbonate platforms during early burial (also termed “Kohout convection”)

draws on an infinite reservoir of magnesium and could lead to large scale geothermal dolomite formation.

4. Due to the lack of Mg2+

and slow reaction rates, mixing zone dolomitization is not a viable mechanism for large-scale

dolomitization in isolated platforms

5. Simulations show the propagation of dolomite fronts by fingering generating patterns of dolomite consistent with those

observed in the limited number of outcrop studies.

6. Dolomite “fingers” tend to connect to the “main body” in a single reflux episode and are rarely isolated “pods”.

7. The salinity of the reflux brine controls anhydrite primary precipitation and cementation, leading to the formation of

important diagenetic features such as bedded anhydrite and anhydrite cement, with the latter subject to remobilization.

8. Dolomite “sweet spots” are located close to the dolomite front, have a variable thickness and are generally well

connected in 3D.

9. Continuous hydrothermal flow over tens of thousands of years is required to lead to large scale HTD formation.

10. The predicted distribution of dolomite (main body and fingers) anhydrite (bedded anhydrite and anhydrite cement), and

high-K dolomite provided important guidance to test the RTM predictions, and to facilitate the development of 3D

reservoir connectivity models with different scenarios for reservoir simulation studies. Future case studies will utilize

well and field data to test the reactive transport models.

When integrated with conventional subsurface data and stratigraphic, geochemical, and structural framework, Reactive

Transport Models of dolomitization provide fundamental and robust predictive concepts and reservoir quality models for

exploration and new / mature field developments. In particular, the state-of-the-art simulations allow the analysis and 3D

visualization of dolomite body spatial and temporal evolution that can translate into alternative “process-based” well

correlation methods and strategies for populating diagenetic bodies and their petrophysical properties in geological models

for reservoir flow simulations.

ACKNOWLEDGEMENTS

We thank contributions from ExxonMobil colleagues Jeremy Jameson, Jon Kauffman, Bob Alway, Scott Parker, Mike

Weaver, and Jim Anderson. Careful reviews by Glenn Goss, Shawn Fullmer and Mauro Lo Cascio significantly improved the

manuscript. We also thank Drs. Tianfu Xu and Karsten Pruess from Lawrence Berkeley National Laboratory for their

generous support in using ToughReact program. ExxonMobil’s permission to publish this paper is greatly appreciated.

IPTC 123456 15

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